fix: [ENG-3025] address review findings on response-file flag PR

Four cleanups from PR review (one per finding):

1. Defer --response-file unlink until after daemon dispatch returns.
   Previously the unlink fired between local validation and dispatch, so
   a transport error (daemon down, malformed payload, daemon-side throw)
   destroyed the envelope file the agent had paid an LLM call to author.
   Now the unlink only fires on a non-throw dispatch (success OR
   validation-failed); a daemon-error throw skips it and the file
   survives for retry. Unlink failure post-success appends a warning to
   the dispatch envelope instead of aborting.

2. Replace `as Error & {kind?: string}` cast on the parse-error catch
   with `instanceof InvalidResponseFormatError`. Required exporting the
   class from curate-session.ts. Matches the existing pattern for
   InvalidResponseFileError and respects the no-`as`-Type convention.

3. Pre-check whitespace-only payload in the command and emit the
   transient `empty-response` envelope directly instead of letting
   JSON.parse('   ') throw and collapse it into the terminal
   `invalid-response-format` kind. Preserves the documented contract
   where `empty-response` keeps the session alive for retry.

4. Replace the one-line buildUnknownSessionEnvelope wrapper with a
   direct export of the internal unknownSessionEnvelope. Same shape,
   one fewer indirection.

Interactive verification: file preserved on whitespace-only payload
(both inline and file), happy-path file deleted post-dispatch, real
envelope still curates clean. 8877 tests pass (1 new for the
parseCurateResponse-throws-InvalidResponseFormatError contract).